1. Field of the Invention
The present invention relates to an imaging apparatus and an integrated circuit that functions as an imaging apparatus, and more particularly, to an imaging apparatus that appropriately captures an image of a scene (subject) having a large dynamic range, an integrated circuit that functions as such an imaging apparatus, and a computer-readable storage medium that stores a program used in such an imaging apparatus.
2. Description of the Related Art
Imaging apparatuses, such as a digital still camera for capturing still images and a digital video camera for capturing moving images, capture images in the following manner. In an imaging apparatus, an optical system focuses light and forms an image through exposure control, and a solid-state image sensor, such as a CCD (charge-coupled device) image sensor or a CMOS (complementary metal oxide semiconductor) image sensor, converts the image to electric signals, which are analogue image signals, by photoelectric conversion. In the conventional imaging apparatus, a circuit that performs signal processing, such as analogue front end processing, then processes the analogue image signals, and an A/D (analogue-to-digital) converter converts the analogue image signals to digital image data. The digital image data is subsequently subjected to image processing, such as video gamma correction (gamma correction with a gamma of 0.45), knee adjustment, luminance conversion, and color difference conversion. The processed digital image data is converted to data in a standardized format. More specifically, when the digital image data is still image data, the data is converted to, for example, JPEG (Joint Photographic Experts Group) data. When the digital image data is moving image (video) data, the data is converted to, for example, MPEG (Moving Picture Experts Group) data or DV (Digital Video) data. The digital image data in the standardized format is recorded onto a recording medium, such as a memory card, a hard disk, an optical disc, or a magnetic tape.
The whitest point of the data defined by the standardized format described above (image or video format), which specifically corresponds to the highest luminance level of the data reproduced on a display, is assumed to have a luminance level of 100%. In this case, the conventional imaging apparatus is normally designed to form an image having a dynamic range of luminance levels of 100 to 200%. The dynamic range of 100% means that signal values (for example, luminance levels) of the image correspond to luminance levels of 0 to 100%. In other words, the minimum signal value corresponds to a luminance level of 0%, and the maximum signal value corresponds to a luminance level of 100%.
A first conventional imaging apparatus compresses the dynamic range of a captured image to eliminate its luminance area exceeding a luminance level of 100% through processing called knee adjustment, and obtains an image having a dynamic range of 100% or less.
The dynamic range of an image that is captured by the conventional imaging apparatus is normally determined by the exposure light amount of the optical system (exposure light amount determined by the aperture or shutter speed) and the electric amplification amount of electric signals, which have been generated by photoelectric conversion.
The conventional imaging apparatus appropriately captures an image (video) of a bright scene (subject) that has a large amount of light either by setting a small aperture or by setting a fast shutter speed. This imaging apparatus appropriately captures such an image because the imaging apparatus is provided with a sufficiently large amount of light. Typically, users of imaging apparatuses may intentionally produce “desirable blur” in a captured image (video), or more specifically intentionally defocus and blur the background of the image (video) by setting a faster shutter speed and a larger aperture and decreasing the depth of field. Even when the depth of field is decreased in that manner, the imaging apparatus appropriately captures an image (video) in environments that have large amounts of light.
Also, users may often set a slower shutter speed and a smaller aperture and increase the depth of field.
In either of the two cases in which the depth of field is increased and decreased, the imaging apparatus is provided with a sufficiently large amount of light when capturing an image of a bright scene (a subject in a bright environment), and thus is not required to perform electric amplification of signals. When capturing an image of a bright scene (subject in a bright environment), the conventional imaging apparatus does not electrically amplify electric signals, which have been generated by photoelectric conversion.
When capturing an image of a dark scene, however, the imaging apparatus is required to maximize the aperture and slow the shutter speed to obtain a sufficiently large amount of light. When the imaging apparatus performs such a long exposure, the imaging apparatus and the subject may move during the exposure. Such movement of the imaging apparatus or the subject may blur images. To prevent blurry images caused by apparatus or subject movement, the conventional imaging apparatus limits the slowest shutter speed setting to the speed at which such apparatus movement does not occur. To compensate for an insufficient amount of exposure light, the conventional imaging apparatus electrically amplifies electric signals, which have been generated by photoelectric conversion. Such signal amplification processing is referred to as “high-sensitivity mode processing” or “push processing”. To prevent the S/N (signal-to-noise) ratio from deteriorating and to ensure the quantization resolution of A/D conversion, the electric push processing is typically performed by amplifying an output of the image sensor of the imaging apparatus using an analogue circuit. Japanese Unexamined Patent Publication No. 2002-135651 describes one technique of push processing, which is performed by switching the gain of an analogue amplifier circuit according to the ISO speed.
When the imaging apparatus obtains image signals using a solid-state image sensor, such as a CCD image sensor or a CMOS image sensor, the image signals have a small dynamic range. In this case, when capturing an image of a scene (subject) that has a large dynamic scene (for example, when capturing an image of a backlit person outdoors or when capturing an image of an outdoor landscape from indoors through a window), the imaging apparatus may fail to appropriately reproduce both of the dark portion and the bright portion of the subject as clear images because the luminance levels of the dark portion and the bright portion of the image greatly differ from each other.
To overcome this drawback, a second conventional imaging apparatus uses a solid-state image sensor that can control the charge storage time. The second conventional imaging apparatus superimposes a subject image formed through a long exposure and a subject image formed through a short exposure, and consequently obtains a subject image that has a large dynamic range (see, for example, Japanese Unexamined Patent Publication No. 2005-72965).
A third conventional imaging apparatus uses a solid-state image sensor that includes high-sensitivity pixels constituting a half of all the pixels of the image sensor and low-sensitivity pixels constituting the remaining half of the pixels. The third conventional imaging apparatus superimposes an image signal corresponding to a high-sensitivity pixel and an image signal corresponding to a low-sensitivity pixel, which have been generated through exposures performed for the same exposure time, and consequently obtains a subject image that has a large dynamic range (see, for example, Japanese Unexamined Patent Publication No. S59-210775).
However, the conventional imaging apparatus (first conventional imaging apparatus) is only designed to capture an image with a dynamic range of up to 200%, and compresses the dynamic range of the captured image to 100% through knee adjustment. When, for example, the conventional imaging apparatus is used to capture an image of a face of a backlit person with a blue sky background, the exposure light amount of the conventional imaging apparatus is set in a manner that the person's face will have an appropriate luminance level of 70%. In this case, the luminance level of highlights that the conventional imaging apparatus can handle without saturation (maximum luminance level) would be as low as about three times the luminance level of the person's face.
However, the dynamic range of a scene (subject) including a clear sky can be easily as high as several hundred percent or more. When the conventional imaging apparatus is used to capture an image of such a scene (subject), the light amount of the image sensor of the imaging apparatus may be saturated. In this case, the imaging apparatus fails to appropriately reproduce a high-luminance portion of the image, such as a clear sky portion.
The second conventional imaging apparatus obtains a subject image that has an increased dynamic range by using a subject image formed through a short exposure. However, the short exposure means a small exposure light amount. With the small exposure light amount, the amount of a signal generated by the solid-state image sensor is small. In this case, the S/N ratio of the image would be low. The third conventional imaging apparatus obtains a subject image with an increased dynamic range by using a signal corresponding to a low-sensitivity pixel. However, the amount of a signal corresponding to a low-sensitivity pixel is small. In this case, the S/N ratio of the image would be low in the same manner.
To solve the above problems, it is an object of the present invention to provide an imaging apparatus, an imaging method, an integrated circuit that functions as an imaging apparatus, and a computer-readable storage medium that stores a program used in an imaging apparatus for capturing an image of a scene (subject) having a large dynamic range (for example, a face of a backlit person with a blue sky background) without saturating a high-luminance portion (for example, a background sky portion) of the captured image.